It all depends on the manifold type. There are three different types of manifolds for flatheads. There is the "Super" where the carbs are placed directly over the intake ports, the "Regular", where they are spaced closer together, but still equidistant from the ends of the manifold. These types need a straight linkage. The third type, which I call "Biased" has the front carburetor placed a smidge forward of the stock location, with another between it and the fuel pump. It is my contention that these were meant to be run with progressive linkage, with the front being the primary. With this set up, the engine runs essentially like a stock two barrel most of the time, with the rear carb cutting in at full throttle to add extra fuel mixture, distribution be damned.
Also, I asked "CharlieNY" about this for my Navarro "Universal" (a "Super" with exhaust heat) and he said to start with 51 main jets and to plug the power valves. I purchased those items from him.
Here is a picture of a "Biased" manifold; note that the front carburetor is very close to the stock location. This type also allows the generator to be mounted in the stock location.